1. Field of the Invention
The present invention relates to an electromechanical transducer (physical sensor) that transduces a mechanical variation into an electrical signal and a method of fabricating the same.
2. Description of the Related Art
An electromechanical transducer that transduces a mechanical variation into an electrical signal is widely known at the position of a physical sensor. The sensor electrically captures a physical change which a material itself has with respect to an environmental change, and various kinds of sensors have been developed in recent years, and their sizes have been reduced.
Various kinds of products to which such sensors are applied are developed and are on the market. It is the challenges how these products are fabricated to be small and light at low cost. Accordingly, a physical sensor that is a component to be a part of the product is naturally required to be reduced in size. However, it is not allowed to degrade the performance thereof even if the physical sensor reduces in size.
Therefore, in order to enhance sensitivity of physical sensors, various kinds of sensors using different methods are conventionally developed. Taking an acceleration sensor as an example, acceleration sensors in recent years are made of semiconductors, and the typical methods include a capacitance type, a piezoelectric type and a piezoresistance type.
The capacitance type is a system for detecting a change in an electrostatic capacitance between an electrode which is fixed and does not move and an electrode which deforms and moves when acceleration is applied thereto, and uses a gap change between the electrodes to cause a capacitance change. The advantages of this system are easiness in correction with respect to a temperature change and high sensitivity. The disadvantage is that since a capacitance change between a pair of electrodes is small, a capacity change has to be increased by performing wire connection to connect a number of electrode pairs in parallel. Since a noise easily enters, a contrivance to avoid receiving an influence of stray capacitance as much as possible, such as placing a detection circuit near to the sensor or the like is required.
The piezoelectric type is a system for detecting electric charges generating due to strain applied to a piezoelectric element by acceleration or the like. The advantages of this system are compactness and light weight. The disadvantages are high output impedance and static acceleration cannot be detected.
The piezoresistance type is a system in which a resistance value of a resistance element changes in accordance with stress applied to a body provided with a piezoresistance element, and the resistance value change is detected as a voltage change by forming a bridge with a peripheral circuit. The advantages of this system are that size reduction is easy, the detection circuit is simple and the like. The disadvantage is that since the piezoresistance element is a semiconductor, change with respect to the temperature is large, and correction is required.
Among these various kinds of systems, the piezoresistance system is compact and simple, and possibility of mass production, and therefore, the piezoresistance system is widely used. What is important in the piezoresistance system is that sensitivity of the piezoresistance itself becomes low as a result of the weight becoming small with reduction in size. It is also important that sensitivity or the like necessary for a desired structure can be adjusted. Various structures and fabrication methods are contrived to adjust the sensitivity (see Patent Document 1).
An acceleration sensor of a prior art disclosed in Patent Document 1 is a semiconductor acceleration sensor in which a weight part which displaces by receiving acceleration is supported with a frame part via a flexible part, and a sensor part by piezoresistance is provided at the flexible part. An insulating film such as a silicon nitride film or the like is formed on a front surface of the flexible part, and film thickness of the insulating film is adjusted in accordance with the sensitivity to be set.
Further, it is a sensor element used as a magnetic resistance sensor, an air flow sensor, an acceleration sensor, a pressure sensor and the like, in which a front surface of a plane-shaped sensing region having a fine line supported by a sensor substrate is coated with a silicon resin film is also known (see Patent Document 2).
Further, it is also known that in the same sensor element as described above, a resin film is interposed between the sensor substrate and the sensing region (see Patent Document 3).
These Patent Documents also describe that an inorganic film such as a silicon nitride film is formed as a passivation film or an interlayer insulating film in the sensor element.
Patent Document 1: JP 2004-354074A
Patent Document 2: WO 01/046708A
Patent Document 3: WO 01/088482A
In order not to reduce sensitivity even if a sensor having a structure in which a sensor part by piezoresistance is provided at a beam part that is a flexible part in a small piece shape and a mechanical variation is applied to the beam part by a weight part supported by the beam part, just like the semiconductor acceleration sensor described in the above described Patent Document 1 is made compact, the beam part is made slim and thin to be easily displaced, and sufficient durability has to be kept. Therefore, it is difficult to obtain required set sensitivity by only adjustment of the film thickness of the insulating film.
Since the sensor elements described in Patent Documents 2 and 3 do not have the structure in which the sensor part is provided at the beam part in a slim piece shape, but have the structure in which the planar sensor region is provided on the sensor substrate, flatness and protective properties of the sensor region can be enhanced by the resin film, but it is difficult to obtain sufficient sensitivity when using them as three-dimensional acceleration sensors.